The rotor blade of a transonic axial flow compressor (such as the one disclosed in U.S. Pat. No. 5,137,419) rotates at a high speed with a suitable gap defined between the tip of the blade and the opposing inner circumferential surface of the outer casing, and the region adjacent to the blade tip is subjected to an extremely complex flow pattern owing to the boundary layers that develop along the surfaces of the outer wall and the blade, the leak flow that flows through the gap defined between the blade tip and the opposing wall surface, and the interferences between these flows. In particular, owing to the interferences between the leak flow produced in the gap between the blade tip and the opposing wall surface and the shockwave produced between adjacent rotor blades, a low momentum region having a certain circumferential expanse is produced behind a rear half of each rotor blade (see FIG. 5), and this not only severely impairs the efficiency of the tip end of each rotor blade but also degrades the surge property of the rotor blade. Furthermore, the egress of such a low momentum region from each rotor blade promotes the development of a boundary layer on a downstream side of the rotor blade, and impairs the aerodynamic property of the stator blade located downstream of the rotor blade.
To eliminate such a problem, it has been proposed to provide a concave surface on the negative pressure side of each rotor blade to redirect the airflow, and to thereby generate a compressive wave upstream of the shockwave (Prandtl-Meyer flow). This reduces the Mach number of the flow directed to the shockwave, and minimizes the shockwave loss. As this measure additionally controls the leak flow in the upstream region of the shockwave where the load on the blade is most pronounced, the leak flow loss is also minimized.
However, according to this prior proposal, a desired result may be achieved only over a certain operating range, but not outside this range because the compressive wave would not be produced as desired outside the limited operating range and hence the loss cannot be reduced to an acceptable extent.